The invention relates to a belt retractor for a vehicle occupant seat belt, comprising a belt reel rotatably mounted in a housing, a torsion bar extending in the interior of the belt reel and connected by its first axial end non-rotatably to the belt reel and by its second axial end non-rotatably to a ratchet disk provided with a ratchet toothing, and a load-bearing ratchet pawl capable of being brought into engagement with the ratchet toothing to block the belt reel.
The torsion bar in the interior of the belt reel serves to limit the maximum force acting in the seat belt, especially when the vehicle is involved in an accident, to a degree preventing injury of a restrained vehicle occupant. Limiting the force is achieved in that as of a critical belt webbing force, rotation of the belt reel relative to the non-rotatably mounted ratchet disk is made possible, as a result of which a specific length of the belt webbing can be dispensed from the belt reel. This dispensed length of belt webbing is available as an additional length for decelerating, more particularly, the trunk of the vehicle occupant.
As regards the trunk, the risk of injury for a vehicle occupant is dictated almost exclusively by the force acting as a whole on the trunk. If only the seat belt is used to restrain the vehicle occupant, the force acting on the trunk results exclusively from the force in the belt webbing. If, however, a gas bag is additionally used, which is deployed in front of the vehicle occupant when the vehicle is involved in an accident, to ensure a controlled deceleration of the vehicle occupant, the load on the trunk as of a specific forward dislocation is composed of the force introduced into the trunk by the seat belt as well as the force introduced into the trunk by the gas bag. So that the overall load acting on the trunk does not exceed a predetermined degree, it would be desirable for the force acting in the seat belt to decrease as of that point in time at which the trunk of the vehicle occupant comes into contact with the gas bag. Preferably, the change in the force counteracting dispensing of the belt webbing should be brought about by as simple means as possible, it being further desirable that the characteristic of the belt webbing force versus released belt webbing length is freely adjustable within broad limits so that the belt retractor can be adapted to the individual mounting conditions in the vehicle.
The invention provides a belt retractor satisfying these requirements. In accordance with the invention it is provided for in the case of a belt retractor of the kind as stated at the outset that the torsion bar comprises a first section as well as a second section, the cross-section of which is smaller than that of the first section, that an arresting portion is configured between the first and second sections and that a bypass element is provided which can be transposed from a starting position connecting the arresting portion non-rotatably to the second end of the torsion bar into a release position in which the arresting portion is rotatable relative to the second axial end of the torsion bar. In this configuration, release of the belt webbing occurs initially only with twisting of the first section of the torsion bar; the second section of the torsion bar is not involved in twisting since it is bypassed by the bypass element. Depending on the respective mounting situation, the bypass element is transposed, following a predetermined rotation of the belt reel relative to the ratchet disk, from the starting position into the release position. At this point, also the second section of the torsion bar is involved in the rotation of the belt reel relative to the ratchet disk. Since the second section has a smaller cross-section. than the first section, a lesser belt webbing dispensing force is needed. The point in time of transposing the bypass element from the starting position into the release position is preferably selected such that it coincides with the start of the diving action of the trunk of the vehicle occupant into the deployed gas bag.
In accordance with a preferred embodiment it is provided for that the arresting portion and the second axial end of the torsion bar are splined and that the bypass element is a bypass sleeve splined complementary in its interior. In this embodiment the bypass element is configured as a sleeve which is particularly simple to fabricate, slidingly arranged on the arresting portion and on the second axial end of the torsion bar. For transposing the bypass sleeve from the starting position into the release position, a variety of mechanisms of simple design may be provided. For example, a positioning cylinder may be employed which is activated by means of a pyrotechnic charge. Preferably, however, the bypass element is transposed from the starting position into the release position mechanically.
In accordance with a preferred embodiment it is furthermore provided for that the first axial end of the torsion bar is also splined and that a positioning sleeve is arranged non-rotatably on the first axial end of the torsion bar, this positioning sleeve being splined complementary in its interior and comprising an end face facing the second axial end of the torsion bar, which end face is configured as a control cam with which the end face of the bypass sleeve facing the first axial end of the torsion bar is in contact. In this configuration a rotation of the arresting portion relative to the first axial end of the torsion bar is directly exploited to transpose the bypass sleeve from the starting position into the release position. By suitably configuring the control cam it can be set as of which rotation of the first axial end of the torsion bar relative to the arresting portion the second section of the torsion bar is involved in the rotation of the belt reel. A limitation exists only to the extent that the release position needs to be attained at the latest after one revolution of the arresting portion relative to the first axial end of the torsion bar.
If more than one revolution of the belt reel is desired before also the second section of the torsion bar is involved in limiting the force, a thread may be configured, for example, on the bypass sleeve in which a counterpiece engages such that a transposition of the bypass sleeve from the starting position into the release position is possible. Depending on the pitch of the thread, the point in time at which the bypass sleeve is transposed into the release position can be set to any desired degree of relative rotation between the arresting portion and the first axial end of the torsion bar.
Advantageous aspects of the invention read from the sub-claims.